Printing of liquid crystal droplet laser resonators on a wet polymer solution and product made therewith

ABSTRACT

A method of manufacturing a security feature for identifying objects or documents of value. The method may include the steps of encoding information in a pattern; and ink jet printing a chiral nematic liquid crystal material from a reservoir using a print head on to a substrate in the pattern. Thus, the method forms a patterned array of chiral nematic liquid crystal material deposits. The print head, or the reservoir, or both, may be heated to a temperature above the clearing point of the chiral nematic liquid crystal material. The chiral axes of the chiral nematic liquid crystal material deposits may be aligned substantially perpendicular to the substrate such that a predetermined portion of the electromagnetic spectrum is selectively reflected over other regions of the electromagnetic spectrum by the chiral nematic liquid crystal material deposits.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/402,292 filed Nov. 19, 2014, entitled “PRINTING OF LIQUID CRYSTALDROPLET LASER RESONATORS ON A WET POLYMER SOLUTION AND PRODUCT MADETHEREWITH,” which claims priority to 35 USC §371 of PCT ApplicationSerial No. PCT/GB2013/051368, filed May 24, 2013, entitled “Methods forManufacturing Photonic Devices,” which claims priority to GB PatentApplication No. 1209235.9, filed May 25, 2012, and GB Patent ApplicationNo. 1214439.0, filed Aug. 13, 2012, which are each incorporated hereinin their entirety by reference.

BACKGROUND TO THE INVENTION

Field of the Invention

The present invention relates to methods for manufacturing photonicdevices and to photonic devices manufactured by such methods. Theinvention has particular, but not exclusive, application to themanufacture of liquid crystal laser devices.

Related Art

Liquid crystal (LC) materials are a class of functional photonicmaterials. LC materials contain molecules which have a tendency toself-organize along an optical axis. The way in which the molecules inLC materials align dictates the optical properties of the LC material.For example, chiral liquid crystals have a tendency to self-organizeinto a helicoidal arrangement around an optical axis. Due to thebirefringence of the material, this helicoidal arrangement results in aperiodic variation of the refractive index along the optical axis. Forsuitable periodicities, this gives rise to a photonic band-gap forvisible wavelengths of circularly polarized light.

The optical properties of chiral LC materials make them suitable forapplications ranging from bistable displays to lasers. Incorporation ofan organic laser dye, as the light harvester or gain medium, into theoptical cavity can lead to laser emission at the photonic band-edges.Laser devices built based on these materials are characterised by verylow cost manufacturing, small size and selectable wavelength ofemission, currently in the range 400 nm to 850 nm (See References [4]and [2]).

Applications of achiral LC materials include conventional flat-paneldisplays (nematic LCDs), variable retarders and SLMs.

US2011/0097557 discloses the manufacture of security features, e.g. forbank notes, in which a polymerisable LC material is printed onto a solidPVA layer. The PVA layer is unrubbed but is found to promote alignmentfor certain LC materials.

Conventional semiconductor lasers used in most modern laser systems aresolid-state devices that are typically manufactured using a complexprocess involving a combination of deposition, etching andphotolithographic steps on high quality single crystal semiconductorwafers. Such conventional lasers cannot be manufactured using relativelylow cost printing technologies such as bar coating or inkjet printing,for example. It has previously been shown in References [11] and [12]that printable emulsion-based LC laser systems can be deposited on awide variety of substrates including, for example, glass, plastic, metalor paper. These documents describe deposition of a LC lasing medium ontosuitable substrates using emulsified samples and a simple bar-coatingprocess. In these disclosures, the liquid crystal and laser dyecomposition were emulsified in a continuous phase of PVA, this emulsionthen being applied to the substrate. Subsequent drying of, andinterfacial interaction with, the continuous phase causes advantageousstresses to be applied to the LC droplets, assisting in the developmentof suitable alignment of the LC molecules.

SUMMARY OF THE INVENTION

The present inventors have found that although the approach described byReferences [11] and [12] provides a simple fabrication process, theapproach has some disadvantages. The individual LC droplets arepolydisperse in terms of size, typically with diameters in the range oftens to hundreds of microns. This has an effect on the quality of thelaser emission. Furthermore, the droplets are locally randomlydistributed in the continuous phase. Therefore the approach used inReferences [11] and [12] does not allow accurate positioning ofindividual droplets, with the consequence that the spatial position ofthe emission of light from the device cannot be accurately controlled.

Accordingly, the present inventors attempted to overcome the abovedisadvantages by depositing patterns of LC lasing medium. The presentinventors found that it is indeed possible to produce a required patternof deposits of LC lasing medium, for example by inkjet printing.However, the present inventors have found that direct deposition of theLC lasing medium onto clean, dry surfaces fails to produce a suitabledegree of alignment of the LC molecules (for example, a suitablealignment for many devices that use chiral LC materials is one in whichthe chiral LC helical axis is aligned perpendicular to the substrate).

Control of alignment within LC materials is known to be important toallow the desired optical properties of the LC material for a particularapplication to be obtained. For example, flat-panel display devicescomprising nematic LC require a uniform direction of the LC directori.e. alignment of the LC material, for the display to function. Forexample, lack of uniform alignment within a deposited LC material foruse as a lasing medium can result in multi-mode laser emission, or evenno laser emission, which is typically undesirable.

The present invention has been devised in order to address at least oneof the above problems. Preferably, the present invention reduces,ameliorates, avoids or overcomes at least one of the above problems.

In a general aspect of the invention, the present inventors have foundthat it is advantageous to deposit a liquid crystal (LC) material onto aflowable layer of material that is substantially immiscible with the LCmaterial. Furthermore, the present inventors have found that furtheradvantages can be achieved when, on impact of the LC material, theflowable layer of material shapes the LC material with the result ofpromoting alignment of molecules in the LC material.

In a first preferred aspect, the present invention provides a method ofproducing photonic devices, such as lasers, or optical features bydepositing liquid crystal (LC) materials in one or more discrete regionson surfaces.

In a second preferred aspect, the present invention provides a method ofmanufacturing a photonic device, the method comprising the steps of:

(i) providing an aliquot of a liquid crystal (LC) material of volume V,volume V being the same volume as that of a sphere of diameter D1; and

(ii) depositing the aliquot onto the surface of a flowable materiallayer to form a liquid crystal deposit, the flowable material and the LCmaterial being substantially immiscible, wherein the liquid crystaldeposit adopts a deformed shape relative to the shape of the aliquot dueto interaction with the flowable material layer, the liquid crystaldeposit having a maximum length L1, measured in a direction parallel tothe flowable material layer surface before deposition, wherein L1 isgreater than D1.

In a third preferred aspect, the present invention provides a photonicdevice obtained by or obtainable by the method of the first or secondaspect.

In a fourth preferred aspect, the present invention provides a laserdevice obtained by or obtainable by the method of the first or secondaspect.

In a fifth preferred aspect, the present invention provides a photonicdevice having at least one liquid crystal (LC) material deposit formedon an underlayer, the underlayer having an underlayer surfacesurrounding the LC material deposit, wherein the liquid crystal deposithas a maximum length L2, measured in a direction parallel to theunderlayer surface surrounding the LC material deposit, and a maximumheight H2, measured in a direction perpendicular to the underlayersurface surrounding the LC material deposit, so that L2 is greater thanH2, wherein the liquid crystal deposit is shaped to project above theunderlayer surface surrounding the LC material deposit.

In a sixth preferred aspect, the present invention provides a use of aphotonic device according to the third or fifth aspect, in which thephotonic device is subjected to illumination by a source ofelectromagnetic radiation and provides a corresponding response which isdetected by a detector or by observation.

Any of the aspects of the invention may be combined with each other.

Any of the aspects of the invention may have any one or, to the extentthat they are compatible, any combination of the following optionalfeatures.

The term liquid crystal (LC) material as used herein describes amaterial containing at least 50 wt. %, preferably at least 75 wt. %,more preferably at least 90 wt. % of at least one chemical compound thatexhibits liquid crystalline behaviour. Optionally the LC materialcontains a plurality of chemical compounds known to exhibit liquidcrystalline behaviour. Preferably the LC material contains elongatedmolecules.

Suitably the LC material is a chiral LC material, that is a materialcontaining at least 50 wt. %, preferably at least 75 wt. %, morepreferably at least 90 wt. % of at least one chemical compound thatexhibits liquid crystalline behaviour and a chiral additive, for exampleBDH-1281. The addition of a chiral additive allows the chirality of theLC material to be controlled by controlling the periodicity of thevariation in refractive index in the LC material. Suitable chiraladditives are described in U.S. Pat. No. 6,217,792 and WO 2011/137986.Suitably the chiral LC material contains less than 10 wt. % of chrialadditive. Preferably the chiral LC material contains about 2 wt. % to 6wt. % of chiral additive. Chiral LC materials have a tendency to alignin a helicoidal arrangement. Chiral LC materials are particularlysuitably for the formation of a band-edge laser. Alternatively thechiral LC material contains a chiral chemical compound known to exhibitliquid crystalline behaviour.

The LC material may be one, or more, of nematic, chiral nematic, smecticor blue phase materials. Chiral nematic materials are particularlypreferred.

Suitably the LC materials form a laser. Preferably the LC materialexhibits photonic band-edge lasing. Alternatively the LC material mayexhibit random lasing.

Optionally the LC material includes a fluorescence dye, a fluorescentlaser dye, a quantum dot, or other light harvester or gain additives,such as Nd:YAG, Ti:sapphire, Cr:sapphire, Cr:LiSAF Er:YLF, Nd:glass, andEr:glass. to allow the LC material to function as a lasing medium.

Suitably the LC material exhibits a nematic phase. A chiral LC materialexhibiting a nematic phase is particularly suitable for the formation ofa band-edge laser. However, these materials may also be used to form arandom laser.

Optionally the LC material exhibits a smectic phase. Chiral smectic LCmaterials are suitable for the formation of a band-edge laser and mayalso be used to form a random laser.

Optionally the LC material exhibits a blue phase I, II or III. Bluephase materials are particularly suitable for the formation of aband-edge laser.

In alternative embodiments, it is not necessary for the LC material toform a laser. This is the case where, for example, it is intended thatthe photonic device is a passive device. Suitable passive devicesinclude, for example, Bragg-like reflectors, where a known portion ofthe electromagnetic spectrum is selectively reflected over other regionsof the spectrum.

Preferably the aliquot of liquid crystal material is provided by inkjetprinting. To overcome the problem of providing accurate spatialpositioning, the present inventors have developed an inkjet-basedprocess which can preferably be used to construct arrays of LC lasers orother photonic devices whilst preserving the desirable emissioncharacteristics.

The method of the present invention may be used to construct patternedarrays of liquid crystal material deposits, for example a pre-designedcomplex two dimensional pattern. The patterned array may also be aregular array of liquid crystal deposits.

The photonic device of the present invention may have a plurality ofliquid crystal deposits in a regular and/or patterned array.

Optionally the liquid crystal deposit is shaped to project above andbelow the flowable material layer (or underlayer surface) surroundingthe LC material deposit.

Optionally the flowable material layer at least partially encapsulatesthe LC deposit. Where, for example, the LC deposit adopts a lenticularshape, the flowable material layer may encapsulate one of the majorconvex surfaces of the lenticular shape.

The additional advantage of using an inkjet process is the flexibilityin not only depositing the laser materials themselves, with controllableemission characteristics, but that further additive processing is madesignificantly easier. Thus, for example, further materials can bedeposited via inkjet printing. The flowable material layer may bedeposited via inkjet printing (before deposition of the LC material).Other materials may also be deposited in this way, such as otherpolymeric layers to assist with or provide protection of the LC materialand/or improve alignment within the LC material.

The length L1 of the liquid crystal deposit is the maximum distanceacross the liquid crystal deposit on the flowable material layer,measured along a straight line parallel to the flowable material layersurface. For example, when the liquid crystal deposit is a substantiallycircular island when viewed in plan view, L1 is the diameter of theisland.

The liquid crystal deposit has a minimum width W1. The width W1 of theliquid crystal deposit is the minimum distance across the liquid crystaldeposit on the flowable material layer, measured along a straight lineparallel to the flowable surface and passing through the centre point(or centroid) of the liquid crystal deposit when viewed in plan view.Here, the surface of the flowable material layer is considered beforedeposit of the LC material deposit. For example, when the liquid crystaldeposit is a substantially circular island when viewed in plan view, W1is the diameter of the island.

The width W2 of the liquid crystal deposit is the minimum distanceacross the liquid crystal deposit on the support layer (or underlayer),measured parallel to the support layer surface (or the underlayersurface) surrounding the LC deposit, in a similar manner to W1. W2 maybe different to W1 due to a transformation of the flowable materiallayer to the support layer (or underlayer).

The length L2 of the liquid crystal deposit is the maximum distanceacross the liquid crystal deposit on the support layer (or theunderlayer), measured parallel to the support layer surface (or theunderlayer surface) surrounding the LC deposit. For example, when theliquid crystal deposit is a substantially circular drop, L2 is thediameter of the drop. L2 may be different to L1 due to a transformationof the flowable material layer to the support layer (or underlayer).

The length L1 can be measured via static image microscopy afterdeposition of the deposit. The length L2 can be measured via staticimage microscopy after the step of transforming the flowable materiallayer into the support layer.

The width W1 can be measured via static image microscopy afterdeposition of the deposit. The width W2 can be measured via static imagemicroscopy after the step of transforming the flowable material layerinto the support layer.

The diameter D1 can also be measured via microscopy. In the case ofinkjet printing, D1 can be measured via video microscopy by capturingimages of the generated drop after generation from an inkjet nozzle butbefore deposition at the flowable material layer. Alternatively, forother deposition processes, D1 can be calculated based on knowledge ofthe volume V of the aliquot of LC material.

The height H1 of the LC material deposit is the maximum height of theliquid crystal deposit measured perpendicular to the flowable materiallayer surface. Here, the surface of the flowable material layer is againconsidered before deposit of the LC material deposit.

The height H2 of the liquid crystal deposit is the maximum height of theLC material deposit measured perpendicular to the support layer surface(or the underlayer surface) surrounding the LC material deposit.

The height H1 and H2 of the liquid crystal deposit can also be measuredvia static image microscopy after deposition of the drop. The height H1and H2 can also be measured using SEM.

To overcome the problem of poor alignment within the LC material, thepresent inventors have developed an approach taking advantage of thefact that certain flowable materials can be identified and selected inwhich the LC materials are not substantially miscible. For examplesuitable LC materials tend not to be miscible in polymer solutions suchas aqueous PVA. This is demonstrated in References [9], [11] and [12],in which an emulsion of the LC material phase in a continuous phase ofaqueous PVA is formed. The present inventors have found that when thedroplets of the LC phase are smaller in volume than the flowablematerial, e.g. the polymer solution, as in the case of small deposits ona flowable material layer, e.g. a wet film, the interfacial tensionstresses and distorts the LC phases (deposits). The effect of this is topromote the desired alignment of the LC molecules in the liquid crystaldeposit.

The present inventors have found that inkjet printing of a LC materialallows for accurate spatial positioning of the LC material on thesurface on which it is printed. Inkjet printing of a LC material onto aflowable material that is substantially immiscible with the LC materialimproves the spatial positioning of LC material on the surface and alsoimproves alignment within the LC material. The substantial immiscibilityof the two materials means that the materials form distinctive phaseswhen placed in contact with one another. Where the LC material deposithas a circular shape in plan view, the interfacial tension between theLC material and the flowable material layer shapes the deposit of the LCmaterial to have a diameter D2 which is greater than the diameter D1 ofa sphere with the same volume V as the aliquot of LC material deposited.This distortion induces alignment of molecules within the LC material.The non-contact and “self-assembly” nature of this method allows forlow-cost and flexible manufacturing of photonic devices on a broad rangeof surfaces. This method also allows for precise control of patterningalong with control over the photonic property of the LC material byinducing alignment in the LC material.

Suitably the volume of flowable material in the flowable material layeris greater than the volume of LC material deposited by inkjet printingin the process of the present invention. Preferably, the depth of theflowable material layer (or underlayer) is greater than the maximumheight H1 and/or H2 of the LC material deposit. More preferably, thedepth of the flowable material layer (or underlayer) is greater than thediameter D1 of a sphere with the same volume V as the aliquot of LCmaterial.

Preferably H2 is greater than 1 μm, preferably greater than 5 μm.Preferably H2 is less than 100 μm. Preferably H2 is in the range 1 μm to100 μm, more preferably 5 μm to 50 μm and most preferably in the range 5μm to 30 μm.

Preferably the ratio of H1 to H2 is not less than 1:1.

Preferably the ratio of H1 to H2 is not more than 50:1.

A deposit of LC material having H2 within the values described above isconsidered to be particularly suitable for use as a laser due to theresultant length of lasing cavity.

Preferably the ratio D1 to H1 is less than 50:1.

Preferably the ratio of L2 to H2 is from 2:1 to 200:1.

Preferably, the ratio of L1 to D1 is not more than 20:1, preferably lessthan 10:1, and most preferably less than 3:1.

Preferably, the ratio of L1 to H1 is not less than 1:1.

Preferably, the ratio of L1 to H1 is not more than 50:1.

Preferably, the ratio of L2 to H2 is not less than 1.1:1

Preferably, the ratio of L2 to H2 is not more than 1000:1.

Preferably the ratio of W1 to D1 is not less than 0.1:1.

Preferably the ratio of W1 to D1 is not more than 20:1 Preferably theratio of W1 to W2 is not more than 1:1.

Preferably the ratio of W1 to W2 is not less than 0.01:1.

Preferably the ratio of L1 to W1 is not less than 1:1.

Preferably the ratio of L1 to W1 is not more than 200:1.

Preferably the ratio of L2 to W2 is not less than 1:1.

Preferably the ratio of L2 to W2 is not more than 1000:1.

The present inventors have found that operating within the rangesidentified above tends to promote alignment of the LC material in the LCmaterial deposit, due to shaping effects caused by interaction with theflowable material layer.

In some embodiments, LC materials are deposited by inkjet printing ontoa liquid layer. The liquid layer here is an example of the flowablematerial layer of the second aspect of the invention.

In some embodiments, the liquid layer is deposited by film coating usinga doctor blade, or bar coating, or roll-coating or inkjet deposition ofeither a continuous wet film (e.g. solution layer), individual drops,groups of connected drops, or multiple drops in the same location.

Suitably the method comprises a step of transforming the flowablematerial layer into a support layer. Typically, this takes place afterdeposition of the LC material drop. Optionally the step of transformingthe flowable material layer includes curing the flowable material layerto form a support layer. The step of transforming the flowable materiallayer into a support layer may involve any chemical or physical processwhich may convert, e.g. a liquid, into a solid or a semi-solid.Optionally the step of transforming the flowable material layer involvescooling (e.g. solidification), solvent evaporation, cross-linking, orpolymerization (such as photo induced polymerization) of the flowablematerial layer. When the flowable material is a solution, transformationinto the support layer may involve evaporation of some or all of thesolvent from the solution. The transformation of the flowable materiallayer into a support layer provides the advantage that the LC materialcan be supported at a desired location within the support layer.Therefore, the support layer preferably prevents movement of the LCmaterial drop from the desired location, e.g. during movement orvibration of the photonic device.

The step of transforming the flowable material layer into a supportlayer may also involve shrinking the flowable material layer. Forexample, when the flowable material is a solution, evaporation of thesolvent to form the support layer may also result in shrinking of theflowable material layer. Shrinking of the flowable material may furtherdistort the drop of the LC material to further improve alignment ofmolecules within the LC material.

The step of transforming the flowable material layer into a supportlayer may also involve distorting the LC material deposit. The maximumlength of the LC material deposit after the step of transforming theflowable material layer into the support layer is L2. The maximum heightof the LC material deposit after the step of transforming the flowablematerial layer into the support layer is H2.

In some embodiments the maximum length of the LC material depositincreases during the transformation of the flowable material layer intothe support layer such that L2 is greater than L1. This further improvesalignment within the LC material.

In some embodiments the maximum height of the LC material depositdecreases during the transformation of the flowable material layer intothe support layer such that H2 is less than H1. This further improvesalignment within the LC material.

Optionally, after the transformation of the flowable material layer intoa support layer, the ratio of the maximum length L2 of the LC materialdeposit on the support layer to the diameter D1 of the sphere having thesame volume V as the aliquot of LC material is preferably less than20:1, more preferably less than 10:1, and most preferably less than 3:1.

Suitably the method comprises the step of curing the LC materialdeposit. The step of curing the LC material may includephotopolymerization of reactive monomers, for example when liquidcrystalline mono- and di-acrylate materials are contained in the LCmaterial or where the LC material itself is polymerizable. Curing thedeposited drop of LC material prevents disruption of the improvedalignment achieved by the method of the present invention, e.g. duringmovement or vibration of the photonic device.

Suitably the flowable material is a liquid. Preferably the liquid is asolution, for example a polymer solution. The liquid may be a colloidalsolution, suspension or emulsion. In some embodiments the flowablematerial layer is a polymer-dispersed liquid crystal layer as describedin References [11] and [12]. In other embodiments, the flowable materialmay be a material which deforms plastically in response to the arrivalof the generated drop, e.g. a gel or a paste.

When the flowable material is a polymer solution, the polymer solutionmay comprise a polymer selected from the group including: PVA;polyurethane; polyamides, e.g. Nylon 6,6; PMMA; polyimides,poly(pyromellitic dianhydride oxydianilines) and polystyrene. Thesolvent may be any suitable solvent, for example: water,dichloromethane, formic acid, acetone, iso-propyl alcohol, toluene,cyclohexane or other organic solvents or derivatives, for example.

Optionally the polymer solution has a concentration from 1 wt % to 30 wt%. More preferably the polymer solution has a concentration from 5 wt %to 20 wt %.

In some embodiments the flowable material comprises a lasing material,for example the flowable material may be an emulsion based lasing mediumas described in References [11] and [12]. When the LC material depositcomprises a lasing material, the flowable material may exhibit the samelasing action as the LC material deposit. Alternatively the flowablematerial may exhibit lasing action of a different type to the LCmaterial deposit.

Preferably the flowable material contains an alignment component topromote alignment in the LC material. The alignment component mayinclude polyimides, surfactants, polymers (e.g. polyvinyl alcohol,polyurethane, polyamides, Nylon 6,6, polymethyl methacrylate orpoly(pyromellitic, polydiandydride oxydianiline)) or derivatives ofthese materials which promote a preferred orientation of the LC.

The flowable material containing an alignment component provides theadvantage of providing chemical control of alignment within the LCmaterial in addition to the physical control by shaping the LC materialdeposit. Therefore the use of an alignment component further improvesalignment within the LC material.

Preferably the flowable material contains an alignment component topromote planar degenerate alignment in the LC material. When the LCmaterial is a chiral LC material, suitably the planar degeneratealignment component causes homeotropic alignment (perpendicularanchoring) of the LC optical axes within the LC material deposit.

The use of a planar degenerate alignment component also further improvesalignment within the LC material. For example, a chiral LC materialcontains molecules which self-organise along a helicoidal axis, asdiscussed above. The shaping of the LC material deposit described aboveimproves the alignment of the helicoidal axes. Providing a planardegenerate alignment component in the flowable material layer causesmolecules at the base of the LC material deposit to align parallel tothe surface of the flowable material layer. This alignment of moleculesat the base of the helicoidal structures causes the helicoidal axes toalign perpendicular to the flowable material layer surface (orunderlayer surface). This is particularly advantageous for band-edgelasing.

Suitably, the flowable material layer is formed on a substrate. Anotheradvantage of the present invention is that the choice of substrate isnot particularly limited. The substrate may be any material onto whosesurface the flowable material may be deposited and which provides asuitable support for the photonic device. The substrate may belight-transmissive or reflective, e.g. to allow illumination of the LCmaterial drop with pumping radiation for operation as a laser. Thereforesuitable substrates include light-transmissive glass andlight-transmissive plastics. The substrate may, for example, be rigid orsubstantially rigid. Alternatively, the substrate may be flexible.

The surface of the substrate may be patterned with structures, such aswells or barriers. The patterned substrate allows further control of thespatial location, or shape of the flowable layer, deposited LC materialand/or protective layers.

Preferably the flowable material layer is deposited by film coatingusing a doctor blade, bar coating, roll-coating or inkjet deposition ofa continuous film, individual drops, or groups of connected drops.Inkjet deposition of the flowable material allows continuous “printing”of LC photonic devices.

Alternatively, the flowable material may be deposited in discreteregions, for example, in the form of either individual deposits orgroups of connected deposits to form lines or other features. Theprovision of discrete regions of flowable material layer allowsasymmetric stress to be induced in the later deposited LC materials toproduce desired optical features.

When inkjet printing is used to deposit the flowable material, thedigital nature of the inkjet deposition method also allows flexibilityof altering the order and location of material deposition, e.g. LCmaterials onto flowable material or vice versa, or creating multi-layerstructures consisting of one or more layers of LC materials and otherflowable materials. The application of the method of the presentinvention described can also extend to a wider range of LC materials,including but not limited to nematic, chiral nematic, smectic, bluephase or any combination of these materials.

Suitably, the method comprises a step of providing a protective layer.Preferably, the protective material layer is deposited on top of the LCmaterial deposit. The protective material may be the same material asthe flowable material. The protective material layer may be transformedinto a protective support layer. Preferably the protective materiallayer and the flowable material layer (or support layer, or underlayer)together totally encapsulate the LC material deposit. The protectivematerial layer may also further shape the LC material deposit to inducefurther alignment within the LC material.

The protective material layer may provide a hydrophobic or hydrophilicsurface. The protective material layer may be an oxygen scavenger orgetter. The protective material layer may be a moisture barrier or apreferential absorber. For example, polyurethane, PVA,polydimethylsiloxane or other silicones may be used. The optional andpreferred features described above for the flowable material layer alsoapply to the protective material layer. For example, preferably theprotective layer contains an alignment component to improve alignment inthe LC material deposit. Suitably protective materials may comprise:PVA, polyurethane, Nylon 6,6, polymethyl methacrylate, polyimides,poly(pyromellitic dianhydride oxydianiline), metal-oxide polymercomposites or derivatives of these materials.

As mentioned above, preferably the flowable material layer has athickness T1 greater than diameter D1 that would be attributed to thevolume V of the LC material aliquot. This allows the flowable materiallayer to shape the deposit of LC material.

Preferably the flowable material has a thickness T1 less than 10 timesthe diameter D1 of the generated drop of LC material. Suitably thethickness T1 is between 10 mm and 10 nm, more suitably between 1 mm and1 μm, and more suitably still between 100 μm and 10 μm. If T1 is toolarge the drop of LC material deposited on the flowable material layermay be moved from the location of deposition by currents caused withinthe flowable material. For example, when the flowable material is asolution, the length of time the flowable material exists in a liquidphase is typically increased as the thickness of the flowable materiallayer is increased. As the solvent evaporates from the solution layerthis may cause a current to flow in the flowable material which maydisturb the position of a drop of LC material on the flowable materiallayer. Therefore control of the thickness of the flowable material layeris advantageous. Control of the curing rate of the flowable materiallayer is also advantageous for the same reasons.

Preferably the ratio of the thickness T2 of the support layer to thethickness of the flowable layer is in the range 1:1 to 0.01:1.

Preferably the flowable material layer has a substantially constantthickness across the layer. Inkjet printing of the flowable materiallayer allows precise control of the thickness of this layer.

Suitably the protective layer has a thickness in the range of 10 nm to10 mm.

Preferably the method comprises providing a second, and optionally athird, and optionally a fourth etc., aliquot of a LC material anddepositing it on a flowable material layer, i.e. steps (i) and (ii) arerepeated. In this way, there can be provided a device having a pluralityof photonic locations such as active photonic locations. Suitably thesecond and optionally further deposits of LC material are deposited onthe same flowable material layer as the first drop but each in adifferent location from the first drop. In this way the method may beused to obtain an array (preferably an ordered array) of LC materialdeposits on the flowable material layer. Alternatively the second, andoptionally third, and optionally fourth etc., deposits may each bedeposited on a different flowable material layer, i.e. a second, andoptionally third, and optionally fourth etc., flowable material layer.In this way the method may be used to obtain LC material deposits onseveral separate flowable material layers. In this case, the flowablematerial layers may be different, e.g. in terms of composition,thickness etc. in order to provide different control to the drops of LCmaterial.

In some embodiments the method comprises generating a second, andoptionally a third, and optionally and fourth etc., aliquot of a LCmaterial and depositing the LC material on a flowable material layer inthe same location as the first LC material deposit. This allows the sizeof the LC material deposits to be controlled.

In some embodiments, when a plurality of aliquots of LC material areprovided, the LC material deposited in different locations may be thesame LC material. Alternatively the LC material deposited in differentlocations may be different LC materials, for example when differentlasing action is required at different locations.

Preferably the ratio of the volume V (measured in μm³) of the aliquot ofLC material to the length L1 (measured in μm) of the LC material depositis in the range 10:1 to 1,000,000:1 μm².

In some embodiments, it is preferred that the flowable material layercomprises two or more distinct layers. This can be advantageous in orderto provide an upper layer that provides a specific desired interactionwith the incoming LC aliquot and a lower layer (or lower layers) thatprovides either a desired interaction with the incoming LC aliquot oranother a desired interaction with the upper layer.

In a sixth preferred aspect, the present invention provides a use of aphotonic device according to the third or fifth aspect, in which thephotonic device is subjected to illumination by a source ofelectromagnetic radiation and provides a corresponding response which isdetected by a detector or by observation.

In relation to the sixth aspect of the invention, there are severalmodes of operation of the photonic device which are contemplated. Thesedepend on the device itself and on the illumination of the device.

Where the photonic device is a laser, the device typically incorporatesa laser dye. The illumination by a source of electromagnetic radiationpreferably provides optical pumping. In this case the source is itselftypically a laser. However, the source may alternatively be an LED suchas a high power LED.

It is possible to operate the laser above threshold. That is, theoptical pumping provided by the source is sufficient to provide lasingin the LC material deposit.

However, it is alternatively possible to operate the laser belowthreshold, by suitable adjustment of the power of the source, the outputspectrum of the source, or by using a different source. In this case,the laser dye may still fluoresce, but the number of photons emitted bythe laser dye is insufficient to cause true lasing. However, the opticaloutput of the device may include characteristics related to theinteraction of the photons emitted by the laser dye and the photonicband gap of the LC material deposit. In this case, therefore,below-threshold operation of the photonic device may be suitable toprovide a characteristic output that can be detected or observed. Inthis manner, below-threshold operation of the photonic device mayprovide a security feature that can be interrogated using a suitableillumination source. Furthermore, other fluorescent chromophores may beused in place of a conventional laser dye. These may include otherfluorescent taggants, dyes or quantum dots, for example, where thenative fluorescence is modified by the presence of the liquid crystalphotonic bandgap.

It is of particular interest to note that the same photonic device canbe subjected to either of the modes of operation mentioned above. Thus,where a suitable source of electromagnetic radiation is available, thephotonic device can be operated above threshold, with the resultant andthe laser output from the device. However, where only a lower powersource of electromagnetic radiation is available, the photonic devicecan be operated below threshold, with the resultant below-thresholdcharacteristic output.

The photonic device need not incorporate a laser or fluorescent dye. Thephotonic device may be used in passive mode. In this case illuminationof the device by ambient light or by light from a particular source(e.g. an LED of known spectral output) can cause selective reflectionfrom the photonic device based on the photonic bandgap. Non-fluorescentdyes, or material which absorbs certain portions of the electromagneticspectrum, may also be added to the mixture to create a characteristicsignature.

In each of these modes of operation, it is preferred that multiplephotonic devices are provided, arranged in a suitable array or pattern.The array or pattern may be ordered, e.g. it may have some degree ofsymmetry or provide a recognisable shape. However, it is not essentialthat the array or pattern is ordered. A truly random or an apparentlyrandom array or pattern can be used. The positional location of thedevices on the substrate and relative to each other can be used toencode information. Such arrays or patterns are straightforward toachieve based on the inkjet printing approach described. The opticalresponse may vary from device to device in the array. The resultantarrayed optical response of the photonic devices, when suitablyilluminated, provides a powerful basis for a security feature foridentifying objects or documents of value.

Further optional features of the invention are set out below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 shows the emission spectrum under optical excitation at awavelength of 532 nm for a deposit of comparative example 1;

FIG. 2 is a schematic view showing the creation of the flowable materiallayer according to an embodiment of the present invention;

FIG. 3A is a schematic view showing inkjet printing of a LC materialaccording to an embodiment of the present invention;

FIG. 3B is an enlarged view captured during microscopic imaging of theprint head shown in FIG. 3A at 0 μs during generation of a deposit of LCmaterial;

FIG. 3C is an enlarged view captured during microscopic imaging of theprint head shown in FIG. 3A at 20 μs during generation of a deposit ofLC material;

FIG. 3D is an enlarged view captured during microscopic imaging of theprint head shown in FIG. 3A at 50 μs during generation of a deposit ofLC material;

FIG. 3E is an enlarged view captured during microscopic imaging of theprint head shown in FIG. 3A at 250 μs during generation of a deposit ofLC material;

FIG. 4A is a scaled high-speed microscopic image showing the deposit ofLC material generated by an inkjet printing head at a reference time of0 ms;

FIG. 4B is a scaled high-speed microscopic image showing the deposit ofLC material impacting the surface of the flowable material layer 0.2 msafter the reference time;

FIG. 4C is a scaled high-speed microscopic image showing the deposit ofLC material on the flowable material layer 0.6 ms after the referencetime;

FIG. 4D is a scaled high-speed microscopic image showing the deposit ofLC material on the flowable material layer 100 ms after the referencetime;

FIG. 4E is a scaled high-speed microscopic image showing the deposit ofLC material on the flowable material layer 500 ms after the referencetime;

FIG. 4F is a scaled high-speed microscopic image showing the deposit ofLC material on the flowable material layer 2 s after the reference time;

FIG. 5A is a schematic cross-section showing the deposit of LC materialon the flowable material layer;

FIG. 5B is a schematic plan view showing the deposit of LC material onthe flowable material layer;

FIG. 6 is a graph showing the laser emission following opticalexcitation at the absorption maximum of the laser dye from a deposit ofLC material deposited according to the present invention; and

FIG. 7 is a graph showing output intensity as a function of excitationenergy for the same LC material as FIG. 6.

FIG. 8 is a graph showing an example reflection band for a chiralnematic LC material and for the same chiral nematic LC material combinedwith a dye.

FIG. 9 shows the spectral response for the sample of Example 5.

FIG. 10 shows the spectral response for the sample of Example 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, AND FURTHER OPTIONALFEATURES OF THE INVENTION

Chiral nematic liquid crystals (LCs) are a unique class of functionalphotonic materials with applications ranging from bistable displays tolasers.^([1] [2]) In these materials, the constituent elongatedmolecules self-organize into a helicoidal arrangement around thehelical, or optical, axis. The resultant periodic variation of therefractive index gives rise to a photonic band-gap for visiblewavelengths.^([3]) This has recently received significant interest inthe context of photonic band-edge lasing,^([2, 4]) since incorporationof an organic fluorescent dye, as the gain medium, into the helicalstructure, or optical cavity, can lead to laser emission at the photonicband-edges. Such systems offer high slope efficiency, greater than 60%,narrow linewidth emission^([5]) and, with the self-organized “soft”periodic structure, broadband wavelength selectivity and tuneability.Typical laser emission wavelengths are in the range 450 nm to 850nm^([6-10]). The present invention exploits the liquid-like propertiesof the chiral LC laser and describes an inkjet deposition approach forthese materials. Such an approach allows simple and arbitrary positionalcontrol of laser sources, incompatible with conventional laserprocessing and fabrication, to realize new classes of functionalphotonic materials and devices.

Lasing LC media offers significant potential for not only reducingmanufacturing cost, but also to form coatings on surfaces, or devices,currently inaccessible to the traditional processing required in thefabrication of semiconductor lasers. Precise and controllablepositioning of the location and size of individual laser deposits wouldsignificantly improve the functionality of the laser coatings anddevices. Ordered laser array structures, for example, would be ofparticular interest in bio-assay arrays, optofluidic applications andnew information displays.

In recent years, there has been increased interest in using directwriting processes such as inkjet printing as flexible fabricationmethods for electronics and biological devices.^([14]) The presentinvention uses a “drop-on-demand”^([15]) inkjet deposition process thatcontrols precisely the aliquot size and allows the formation ofspatially localized arrays of LC material deposits, for examplespatially localized laser sources. By depositing the LC lasing materialonto a flowable material, e.g. a wet, solution-based polymer, thenecessary alignment within the LC material can be obtained.

When the LC material contains a laser dye, following optical excitationat the absorption maximum of the laser dye, single-mode laser emissionis observed with a well defined threshold and narrow linewidth when thenecessary alignment within the LC material is obtained. The presentinvention shows that the inkjet deposition process has only a minoreffect on the lasing threshold and emission characteristics of the LClaser system relative to control cells fabricated using conventionalglass cell assembly methods. The results demonstrate the possibility ofcreating truly two-dimensional laser arrays of controlled and arbitrarysize, position, and wavelength for use in a diverse range ofapplications.

Although a central aim of the development of the present invention hasbeen to manufacture laser devices, it is not essential that the finaldevice is a laser device. Other photonic devices are contemplated. Otheroptical effects can be generated, enabled by the alignment of chiralnematic liquid crystals.

The inherent chiral nematic liquid crystal optical photonic bandgap,also known as the selective wavelength reflection band, can also be usedby itself to create optical effects and photonic devices, e.g. withoutthe need to add a laser dye, or even with the addition of a laser dyebut without above-threshold operation of the laser dye.

One of the key properties of aligned chiral nematic liquid crystals isthis well-defined one-dimensional photonic band-gap for lightpropagation parallel to the helical axis. In order to observe thephotonic band-gap effect, the intrinsic pitch of the chiral nematicliquid crystal (i.e. the distance for 360° rotation of the local nematicdirector or preferred orientation) should be of the same order as thewavelength range of interest. This optical property of chiral nematicliquid crystals is well known in the literature (see, for example, H. J.Coles, “Handbook of Liquid Crystals” Vol. 2A (Chapter 4) “Chiralnematics: Physical properties and applications” pages 335-411, EditorsD. Demus, J. Goodby, G. W. Gray, H.-W. Spiess, V. Vill, Wiley (1998)).

An example reflection band shown in FIG. 8. In this case, 3.9% w/w ofthe high-twisting power chiral additive BDH-1281 was added to the liquidcrystal host BL006. The mixture was capillary filled into a test cellcomprising glass substrates, separated by 9 μm spacer beads and wherethe surfaces were treated (rubbed polyimide) to obtain helical alignmentperpendicular to the substrates. The cell was then mounted on amicroscope (Olympus BX-51), illuminated with white light, and thecharacteristics of the transmitted light measured by spectrometer (OceanOptics USB2000).

The position of the central wavelength of the photonic band-gap, λ_(c),and width of the reflection band, Δλ, is determined by the intrinsicpitch, P (the length scale at which the LC director rotates by 360°) ofthe liquid crystal and the birefringence, Δn, of the nematic liquidcrystal host, given by the following relations:

λ_(c)c=n_(av)P and Δλ=ΔnP

where n_(av) is the average of the refractive indices parallel andperpendicular to the local nematic director. Through choice of the pitch(readily manipulated through concentration of the chiral additive)and/or birefringence, the position and width of the reflection band canbe easily adjusted.

The 1-D photonic band-gap only exists for light propagating parallel tothe helical axis. Therefore, to observe the photonic band-gap for chiralnematic liquid crystals where the viewing direction is substantiallyperpendicular to the substrate, the chiral axis should be alignedsubstantially perpendicular to the substrate also. The preferredembodiments of the invention, described below, promote such alignmentthrough a print deposition process. In some embodiments, it may beuseful to have certain values of the pitch and/or birefringence suchthat certain wavelengths, or ranges of wavelengths, are preferentiallyreflected. For example, for certain effects it may be advantageous tohave different regions reflecting red, green, or blue portions of thespectrum, or regions outside of the visible spectrum. Preferably, thereflection band may be intentionally designed to reflect a known portionof the spectrum created by a device equipped with an LED light source(e.g. mobile phone, camera phone, smart phone) where otherwise thematerial possesses only low visibility to the unaided eye. In principle,any portion or part of the optical spectrum may be selectivelyreflected.

It may also be practically useful to add absorbing dyes to the liquidcrystal host in order to further modify the absorption characteristics.Also shown in FIG. 8 is a sample in which 1% PM-597 dye was added to thehost chiral nematic liquid crystal. The optical characteristics measuredare essentially a superposition of the dye absorption and liquid crystalreflection band. Many choices of dye would be obvious to those skilledin the art; in particular for authentication and security applicationsit may be beneficial to add dyes which absorb outside of the visiblespectrum, for example.

The optical effects described are particularly applicable in creatingunique optical signatures for anti-counterfeiting, brand authenticationand general security printing and packaging, for example.

In addition to the passive optical reflection described above, otherpractically useful photonic effects may be generated. These includepre-threshold laser emission or fluorescence modified by the presence ofthe chiral nematic photonic band-gap, for example. Further details areprovided in Examples 5 and 6, below.

The liquid crystal (LC) material used in the following examples wasprepared by adding 4.2 wt % of the chiral additive BDH1281 (Merck KGaA)to the achiral nematic LC BL006 (Merck KGaA) to generate the chiralnematic phase (BL006 is a commercially available, wide temperaturenematic liquid crystal mixture comprising4-cyano-4′-pentyl-1,1′-biphenyl and terphenyl derivatives). The highquantum efficiency laser dye, Pyrromethene-597(1,3,5,7,8-pentamethyl-2,6-di-t-butylpyrromethene-difluoroboratecomplex, obtained from Exciton, and used without further purification),was added to the chiral nematic mixture at a concentration of 1% w/w.Mixtures were placed in an oven for a period of 24 hours at 10° C. abovethe nematic to isotropic transition temperature to ensure sufficientthermal diffusion of the constituents. In order to confirm the positionof the long-wavelength photonic band-edge, which defines the laserwavelength of the LC deposit, mixtures were capillary filled into 10 μmthickness glass cells, which had antiparallel rubbed polyimide alignmentlayers.

Comparative Example 1

Initial experiments were performed depositing the lasing LC formulationsonto cleaned, plain glass substrates. The optimized lasing LC mixturecontained the nematic liquid crystal BL006, high twisting power chiraladditive (4.2% wt BDH-1281) and fluorescent dye (1% wtpyrromethene-597). The mixture was designed to have an emissionwavelength at the gain maximum of the dye, close to 585 nm in LC media.The viscosity of the LC mixture was around 110 mPa·s at 20° C.,significantly greater than the jetting limit of 20 mPa·s, suggested bythe print head manufacturer (MicroFab). However, extended rheologicalmeasurement of the LC mixture has shown that its viscosity decreasessignificantly at elevated temperature, obeying the typical Arrheniusbehaviour. While commercial inkjet systems typically process inks atroom or modestly elevated temperature, much higher ink temperature hasbeen shown to be feasible for printing functional materials such asphase-change resists.^([16]) Therefore, the print head was heated to 90°C. to 95° C., close to the isotropic to nematic transition point of theLC laser mixture, to provide the optimum viscosity for printing. Afterprinting, uniform sessile drops were obtained with a typical diameter ofapproximately 200 microns.

A deposit obtained after inkjet deposition onto the cleaned, plain glasssubstrate was examined between crossed polarizers. It was clear thatdisclination lines, representing defects in the director orientation,were widespread across the droplet. Non-uniformity was also visiblewithin the deposit, in this case a substantially circular drop,indicated by a change in colour from the center of the drop to theedges. The colour of the drop, when viewed under cross polarizers wasred at the centre with the colour changing to blue towards the edge ofthe drop as the drop thickness reduces.

To examine the emission characteristics, samples were optically excitedby the second harmonic of an Nd:YAG laser (532 nm, 3-4 ns pulseduration), focused to a spot size of 110 microns. The resultant emissionprofile, shown in FIG. 1, demonstrates a strong multi-mode lasingoutput, characterized by a series of variable linewidth peaks betweenapproximately 560 nm and 620 nm (corresponding to the fluorescenceemission curve of PM-597). The large number of lasing modes isindicative of multiple domains within the droplet, consisting of regionswith different values of the helical pitch.

Previous work, in rubbed planar surface aligned LC cells, described byMorris et. al, [2005]^([13]) showed that multi-domain samples withslightly different pitch values, and with a typical domain size equal toor less than the pump spot size, resulted in multi-mode lasing output.On the other hand, monodomain samples exhibited high quality, singlemode lasing. Poor emission characteristics, such as those presented inFIG. 1, significantly limit the scope of laser applications, whichtypically demand narrow linewidths centered on a well-defined emissionwavelength.

Example 1

10 wt. % polyvinyl alcohol PVA (average molecular weight 10,000 amu, 85%hydrolysed) solutions were drop-casted onto clean glass slides to formwet PVA films. 50 μm-thick polyimide (Kapton) tapes were laid down onthe glass slide first as depth gauges before the PVA solution wasdeposited using a second glass slide as a squeegee. A custom printingrig, consisting of a single-nozzle Microfab printing device (80 μmnozzle diameter) was used to pattern the LC deposits onto the wet PVAfilm. To reduce the viscosity of the LC mixture from 110 mPa·s at roomtemperature to the jettable limit of 20 mPa·s of the MicroFab device,the print head was heated and maintained at between 90° C. and 95° C.,just below the isotropic to nematic transition temperature. A custompneumatic/vacuum controller was used to maintain the LC meniscusposition at the nozzle and a bipolar waveform was applied to eject LCmaterial onto the wet PVA film.

In an attempt to combine the desirable features of inducing alignmentwithin the LC material, for example the alignment necessary to obtainsingle-mode laser emission characteristics, with accurate spatialpositioning of the LC material, the present inventors have developed thealternative deposition approach described above in which the LC materialis directly printed mixture onto a flowable material layer, for examplea wet film of 10 wt % PVA polymer solution in deionized water, asillustrated in FIGS. 2 and 3.

FIG. 2 shows a schematic diagram illustrating the deposition of theflowable material 202, in this case the PVA polymer solution describedabove, on a substrate 200, in this case a glass slide. The substrate isprovided with a depth gauge 204, in this example Kapton tape is providedas the depth guage along opposite sides of the glass slide 200. Theflowable material 202, e.g. the PVA polymer solution described above, isdeposited on the substrate by any suitable method, for example bydrop-casting as described above. A bar or blade 206, for example a glassslide, is then drawn across the substrate in the direction shown byarrow 210 to leave behind a flowable material layer 208 with constantthickness. The thickness of the flowable material layer (the wet film)is defined by the depth gauge 204 and in this case was approximately 50μm across the flowable material layer. This method of depositing theflowable material layer is a method known as doctor blading.

FIG. 3A shows a schematic diagram illustrating inkjet printing of a LCmaterial on a flowable material layer deposited as described in FIG. 2.The features that were described for FIG. 2 are not described again butare given similar reference numbers. FIG. 3A shows a print-head 300which generates aliquots, in this example drops, of a LC material anddeposits LC material on a flowable material layer 208. The print head iscontrolled by any suitable control means, for example piezoelectriccontrol means, to accurately position a LC material deposit on theflowable layer. In this example, the print head inkjet prints an orderedarray of LC material deposits. FIGS. 3B to 3E show an enlargedcross-section through the print head tip 302 of print head 300 as adeposit of LC material is generated and deposited as LC material deposit304 on the flowable material layer. FIG. 3B shows the print head tipbefore generation of a drop of LC material (at 0 μs). FIG. 3C shows theprint head tip at 20 μs after generation of the drop of LC materialbegins. FIG. 3D shows the print head tip at 50 μs after generation ofthe drop of LC material begins. FIG. 3E shows formation of the drop ofLC material 250 μs after generation of the drop of LC material begins;this drop is then deposited onto the flowable material layer 208.

The key stages of a typical droplet deposition event are shown in FIGS.4A to 4F, which show images captured from a high-speed camera. FIG. 4Ashows an approximately spherical drop of LC material 400 approaching theflowable material layer 402 after generation of the drop by inkjetprinting. FIG. 4B shows the LC droplet impacting the surface of theflowable material layer 402 at 0.2 ms after the image shown in FIG. 4A.As the LC material drop impacts the surface the subsequent deformationof the surface of the flowable material layer and the droplet isevident. However, in FIGS. 4C to 4E, frames at 0.6 ms, 100 ms and 500 msrespectively after the image shown in FIG. 4A, it is clear that thesurface tension and immiscibility of the wet PVA solution to the LCdroplet is sufficient to prevent the droplet entering the bulk polymersolution. Finally, in FIG. 4F, the 2 s frame, the droplet is shown inthe equilibrium position on the surface of the film with a well-definedand symmetrical profile.

The necessary alignment of the LC, in the standing helix configuration,for lasing appears to be achieved through a combination of interactionof the PVA polymer with the LC and mechanical forces occurring throughdeformation of the LC droplet. The interaction of PVA polymer with bothnematic and chiral LCs has been examined previously in the context ofpolymer dispersed liquid crystal devices (PDLCs).^([17]) It wasdetermined that PVA promotes parallel arrangement of the LC director atthe interface,^([18, 19]) Following the impact process depicted in FIGS.4B to 4E, and the resultant lateral shear as it reaches an equilibriumstate, shown in FIG. 4F, the LC droplet adopts an oblate shape with theminor-axis perpendicular to the PVA film. The boundary between the LCmaterial and the flowable material on the surface of the drop of LCmaterial encapsulated by the flowable material is shown by the dottedline 404. The ratio of the length L1 of the LC material deposit (in thisexample the deposit was a substantially circular drop, therefore thelength L1 is the diameter of the deposited drop) on the flowablematerial layer in FIG. 4F to the diameter D1 of the sphere with the samevolume V as the aliquot of LC material generated (in this example thealiquot of LC material was a substantially spherical drop of diameterD1) in FIG. 4A is approximately 2:1 (D1 was measured to be 80 μm, L1 wasmeasured to be 160 μm and H1 was measured to be 51 μm). When theflowable material layer and LC material deposit were dried the ratio ofthe length L2 to D1 was approximately 3:1 (L2 was measured to be 250 μmand H2 was estimated to be 10 μm). It is noteworthy that the dropletdoes not continue to wet the surface and both the droplet shape andprofile remain fixed after the film has dried. The combination of theparallel anchoring and lateral motion leads to the standing helixalignment depicted in FIG. 5 and confirmed through polarizingmicroscopy.

FIG. 5A shows a schematic cross sectional view of substrate 500 whichsupports a flowable material layer 502 with thickness T1 on which asubstantially circular deposit of LC material 504 is formed. FIG. 5shows the oblate shape of the deposit of LC material formed according tothe method of the present invention. The deposit of LC material has aheight H1 and a maximum length L1 (diameter in this case). In thisexample, the LC material is a chiral LC material which contains elongatemolecules 506. The immiscibility and the interfacial tension between theLC material and the flowable material induce helicoidal alignment in theelongate molecules.

FIG. 5B shows a schematic plan view of a flowable material layer 502 onwhich a LC material deposit 504 is formed. The LC material deposit shownin FIG. 5B has an elliptical shape with a maximum length L1 and aminimum width W1.

The array of LC material deposits of the present invention, produced asdescribed above for Example 1, were examined between crossed polarizers.Compared to the deposited drop onto the untreated surface describedabove in Comparative Example 1, the LC material deposits producedaccording to the present invention possess greater uniformity than theLC material deposit on a clean glass substrate as described inComparative Example 1. All of the LC material deposits depositedaccording to the present invention were red in colour across the wholedeposit when viewed between crossed polarizers. This is directlyattributable to a more uniform chiral nematic pitch across the depositcompared to the result described for Comparative Example 1. The textureof the deposits produced according to the present invention was found toremain invariant under rotation by 45°, when viewed between crossedpolarisers, indicating that the LC profile is rotationally symmetricwithin the droplet itself. Furthermore, there is no optical extinctionwithin the droplet, i.e. there exist no regions in which the LC directoris parallel or perpendicular to the polarizer or analyzer. Combined withthe fact that the material is chiral, wherein the locally uniform liquidcrystal director precesses to form a macroscopic helix, theseobservations suggest that the likely LC director profile is one in whichthe helical axis is perpendicular to the substrate (Grandjean texture orUniformly Standing Helix). Such an orientation is a pre-requisite forsingle-mode photonic band-edge lasing in chiral LCs normal to thesubstrate, where the laser emission occurs along the helical axis.^([2])

Following the observation of the required chiral LC alignment, theemission characteristics of the LC lasing deposits were examined.

To measure the excitation laser threshold and the polarization of theemission from the pyrromethene-doped LC emulsion samples, coated filmswere photo-pumped by the second harmonic (wavelength=532 nm) of anneodymium yttrium aluminium garnet (Nd:YAG) laser (Polaris II, New WaveResearch), which had a 3-4 ns pulse duration and a repetition rate of 1Hz. The input energy was modulated by a built-in attenuation system andmonitored using a pyroelectric head connected to a calibrated energymeter. In both cases, so as to ensure that the pump beam did notinteract with the photonic band gap, the linear polarization wasconverted to circular polarization of the opposite handedness to thehelix of the chiral nematic LC using a quarter-wave plate. The pump beamwas then focussed to a spot size of 110 μm at the sample using abi-convex lens. The output from the LC samples was collected in theforward direction of the substrates (parallel to the axis of the helix)and focussed onto an HR2000 universal serial bus (USB) spectrometer(Ocean Optics, resolution 0.3 nm) using a lens combination consisting ofa doublet and meniscus lens. To avoid detection of the pump beam, longpass filters were inserted before the detector to remove the output fromthe Nd:YAG laser.

After optical excitation at 532 nm, the resulting emission spectrum andinput-output characteristics are presented in FIGS. 6 and 7. In FIG. 6,the sample shows clear single-mode behavior with an emission peak of 580nm, corresponding to the long-wavelength of the photonic band-edge and alinewidth of less than 1 nm. FIG. 7 shows a plot of the input as afunction of the output energy, the sample exhibits a lasing threshold ofapproximately 300 nJ/pulse. For conventional (non-jetted) samples,filled by capillary action into 10 μm transmissive test cells preparedwith anti-parallel alignment layers, the threshold was measured to be100 nJ/pulse. The main reason for the increased threshold is likely tobe improper matching of the laser spatial profile with the droplet,meaning there is some incident light that is not being usefullyin-coupled into the droplet.

The optimal height H2 for the LC material deposits produced according tothe method of the present invention, for lasing near the gain maximum,is around 10 μm as discussed above.^([20]) The polarization state of theLC laser was experimentally determined and found to be right-circularlypolarized, matching the handedness of the helicoidal structure. Thisprovides further evidence that the laser mechanism is due to the largedensity of states at the edge of the photonic band gap.^([21]) Thesingle mode nature of the lasing output would appear to be a directconsequence of the significant improvement in droplet uniformitygenerated by this inkjet deposition technique.

Example 2

The inventors consider that it is practically useful and advantageous tocontrol the viscosity, and other properties, such as surface tension, ofmaterials and inks designed to be used in printing. This is particularlytrue in inkjet printing where a typical viscosity requirement forsuccessful printing, or jetting, is under approximately 20 mPa·s, withsurface tension ideally around 20-70 mN/m. See the discussion at theURL: [http://www.microfab.com/images/pdfs/microjet_mf4] accessed 16 May2013.

A convenient method to create suitable conditions for inkjet printing isto heat the printhead and/or reservoir, such that the viscosity andsurface tension of the material to be printed is within a suitablerange. For many materials, including liquid crystals, the viscosity hasa strong dependency on temperature; the viscosity typically follows anArrhenius-type or exponential dependency on the temperature of thematerial. However, it is useful to be able to jet at temperatures asclose to room temperature as possible since this reduces the need forcomplex heating elements in the printhead and allows off-the-shelfequipment to be used, resulting in lower cost and more widely availableproduction equipment.

For liquid crystal materials, one particularly suitable way to reduceviscosity at a given temperature, or to lower the temperature at whichsuccessful printing may take place, is to lower the transitiontemperature at which the liquid crystal material, or mesophase,undergoes a phase transition to the ordinary, isotropic, liquid. It iswell known in this technical field that the phase transitiontemperature, or clearing point, can be controlled over a wide range(e.g. from well below 0° C. to over 200° C.) by the formulation of themixture and choice of individual components of the LC material.

In this Example 2, 3% of the chiral dopant R-5011 (Merck) was dissolvedinto the LC E-100 (Merck, Germany). The clearing point of the mixturewas around 68° C. The mixture was inkjet printed using a custom printingrig, consisting of a single-nozzle Microfab printing device (80 μmnozzle diameter) which was used to print the LC deposits. The LC wasdeposited onto a wet PVA (10% PVA in deionised water; PVA was 10,000 amuaverage weight and 85% hydrolysed) film. The wet film was depositedusing a standard K-bar bar coater (RK Print Ltd. UK), with differentfilm thicknesses of 6, 24 and 100 μm.

The standing helix alignment was confirmed through optical observation.To reduce the viscosity of the LC mixture to the jettable limit of 20mPa·s of the MicroFab device, the print head was heated and maintainedat approximately 77° C., which is above the phase transition point ofaround 68° C. A custom pneumatic/vacuum controller was used to maintainthe LC meniscus position at the nozzle and a bipolar waveform wasapplied to eject LC material onto the wet PVA film.

This Example therefore demonstrates that it is possible to reduce theprinting temperature of the LC material by about 20° C. compared toExample 1, by suitable control of the composition of the LC ink.

Example 3

After deposition of the chiral LC has been deposited, it is practicallyuseful to be able to cross-link the LC material. This improves theenvironmental and mechanical ruggedness of the device, and/or allows theaddition of further functionality to the device.

To allow cross-linking of the LC material, a suitable quantity ofreactive mesogen (a material that is a liquid crystal but which containsreactive chemical groups such as acrylate groups as part of the LCmolecule itself to allow joining/cross-linking) is included into thechiral LC mixture. In principle the concentration of the reactivemesogen can be from above 0% up to 100% (100% represents the situationwhere all the LC molecules present have cross-linkable groups).

In Example 3, 3.2% of the chiral dopant R-5011 was dissolved intoUCL-011-K1 (Dai-Nippon Ink Corporation, Japan). The materialsuccessfully jetted at print head temperature of 115° C. to be depositedon wet PVA films of thickness between 24-50 μm.

The PVA films were formed as described for Example 1. The depositedmaterial was then UV cured (365 nm, Omnicure S1000, 10 mW/cm²) for 10minutes.

The standing helix alignment was confirmed through optical observationin a similar manner as for Example 1.

Example 4

Experimental work has been carried out to assess the effect of timingbetween formation of the wet PVA film and subsequent deposition of theLC aliquot by inkjet printing. The amount of time between formation ofthe wet PVA film and subsequent deposition of the LC aliquot by inkjetprinting is referred to here as “processing time”.

Based on this work, there appears to be a preferred lower limit ofprocessing time only after which successful (i.e. standing helix)alignment of the LC is found to be generated.

The minimum value of the processing time is found to vary with wet filmthickness, composition and processing conditions.

For a 24 μm thick wet PVA (10% PVA in H₂O) film, the minimum processingtime is just under 250 seconds. The minimum time reduces as the startingthickness of the wet film reduces. It is observed that the minimumprocessing time can be further controlled (reduced) by active drying ofthe substrate.

For thick or relatively dilute flowable material layers, it is typicallynecessary to employ either a relatively long processing time or takeactive measures (such as active drying) to reduce the processing time.

Without wishing to be bound by theory, the inventors consider that thisphenomenon may be due to the need for the flowable material layer tosettle and reach a suitable concentration (by drying) in order toprovide suitable conditions to promote alignment of the LC material.

The droplet alignment before and after the minimum processing time for aparticular film thickness, composition and processing conditions can bedirectly visualised with polarising optical microscopy as discussedabove in relation to Example 1.

Comparative Example 2

To try to improve the deposit uniformity further, experiments were alsocarried out using deposition onto surfaces treated with rubbed and bakedpolyimide alignment layers only, which promote planar anchoring of theLC in conventional glass cells. In these experiments, the LC materialused was as described above and this material was deposited usingsimilar inkjet processing conditions to those described above. Insteadof the flowable material layer of Example 1, the LC material wasdeposited onto a substrate comprising a planar alignment agent (Merck AM4276) with uniaxial rubbing direction. In this case, significant wettingof the surface by the droplet was observed both immediately afterdeposition and as a function of time, making the devices impractical.

Example 5

A chiral nematic, dye-doped liquid crystal mixture was made, consistingof 4.15% w/w BDH-1281 dissolved in the nematic liquid crystal BL006(both obtained from Merck GmbH, Germany), to which 1% w/w of PM-597laser dye (Exciton, USA) was added. The mixture was capillary filledinto a test cell, with two plane-parallel glass surfaces each coatedwith rubbed polyimide alignment layers, separated by 9 micron spacerbeads, to promote standing helix, or Grandjean, alignment of the chiralnematic liquid crystal. The alignment was confirmed through polarisingoptical microscopy observation. The sample was then optically pumped bya 532 nm Nd:YAG laser (CryLas, GmbH; focussed by a lens to a spot sizearound 100 microns) with a pulse energy of approximately 270 nJ. Theoptical emission was then recorded using an Ocean Optics USB2000fibre-coupled spectrometer. By way of comparison, a 1% w/w PM-597 inachiral BL006 sample (i.e. no chiral additive) without a photonicband-gap, was optically pumped under the same conditions to illustratethe fluorescence observed without the modifying effect of the opticalbandgap. The results are shown in FIG. 9.

As seen in FIG. 9, the presence of the photonic band-gap modifies thefluorescence relative to the achiral (no photonic band-gap) sample. Thisincludes, for example, the creation of local maxima in intensitysuperimposed on the fluorescence. Note that the device here operates inpre-threshold mode, and so can be operated at low intensity of opticalpumping. This means that the pump source can be, for example, an LEDsuch as a flash LED typically provided on a camera phone. Such operationis the subject of Example 6, below.

The LC material used in this example is suitable for inkjet printingonto a flowable material layer, for the formation of discrete LCmaterial deposits as described in other examples above.

Example 6

A mixture containing 3.5% w/w BDH-1305 (chiral dopant, obtained fromMerck GmbH), 1% DCM laser dye (Exciton, USA) in the nematic liquidcrystal host E49 (Merck, GmbH) was filled into a test cell (10 μm pathlength, rubbed polyimide alignment layers). The sample was then pumpedby continuous working LED (450 nm emission wavelength; 1 W opticalpower; obtained from Luxeon) and the optical emission characteristicsmeasured.

The emission intensity as a function of wavelength is shown in FIG. 10where a characteristic profile is shown. The characteristic emissionprofile, in terms of the spectral location and intensity, can becontrolled readily by altering one or more of, for example: the positionand width of the photonic band-gap; the fluorescence spectrum of thedye; the pump wavelength; and the power of the excitation source.

The same optical effects are also observable in samples where printdeposition is used—once the step of standing helix alignment,perpendicular to the substrate, is generated.

CONCLUSION

It is expected that complex and functional laser/photonic device arrays,created by the inkjet technique of the present invention, will haveimportant potential in a variety of technological areas. The combinationof the high degree of positional control, achieved through the inkjetdeposition process, and control of the lasing emission characteristics,continuously selectable in the range 450-850 nm with very narrowlinewidths,^([8]) permit further applications of the technology. Arraysof ink-jet printed LC lasers can also be combined with array-basedpumping techniques^([22]) for the generation of multiple simultaneouslasers, of arbitrary wavelengths, within a single substrate. Ofparticular interest are security applications as described above, andlab-on-a-chip applications such as fluorescence tag-based bio-assays,for example, whereby arrays of independently configurable lasers can beprinted into sample wells for simultaneous optical analysis.

The present inventors have demonstrated that the method of the presentinventions can be used to create reproducible multiple low thresholdsingle-mode laser devices by using precision inkjet deposition of a LCmaterial, for example a LC lasing medium, onto a flowable materiallayer, for example a wet, solution-processible PVA film. Lasers printedin this way retain all the emission characteristics of samples confinedwithin conventional glass cells that are pre-treated with rubbedpolyimide alignment layers but with the simplicity and advantages ofinkjet printing. A combination of interfacial interaction, promotingplanar alignment of the LC director, and shear forces originating duringthe deposition process promote the standing helix alignment required forphotonic band-edge lasing to occur normal to the substrate, e.g. theglass substrate, on which the flowable material is deposited.

While the invention has been described in conjunction with the exemplaryembodiments described above, many equivalent modifications andvariations will be apparent to those skilled in the art when given thisdisclosure. Accordingly, the exemplary embodiments of the invention setforth above are considered to be illustrative and not limiting. Variouschanges to the described embodiments may be made without departing fromthe spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.

Non-patent literature referred to in the description:

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1. A method of manufacturing a security feature for identifying objectsor documents of value, the method comprising: encoding information in apattern; and ink jet printing a chiral nematic liquid crystal materialfrom a reservoir using a print head on to a substrate in the pattern toform a patterned array of chiral nematic liquid crystal materialdeposits, wherein the print head, or the reservoir, or both, is heatedto a temperature above the clearing point of the chiral nematic liquidcrystal material, and wherein the chiral axes of the chiral nematicliquid crystal material deposits are aligned substantially perpendicularto the substrate such that a predetermined portion of theelectromagnetic spectrum is selectively reflected over other regions ofthe electromagnetic spectrum by the chiral nematic liquid crystalmaterial deposits.
 2. The method according to claim 1, wherein thepattern is a two-dimensional pattern.
 3. The method according to claim1, wherein the print head is heated to a temperature above the clearingpoint of the chiral nematic liquid crystal material.
 4. The methodaccording to claim 1, wherein the reservoir is heated to a temperatureabove the clearing point of the chiral nematic liquid crystal material.5. The method according to claim 1, wherein the print head and thereservoir are heated to a temperature above the clearing point of thechiral nematic liquid crystal material.
 6. The method according to claim1, wherein the predetermined portion of the spectrum is selected toreflect a known portion of the spectrum created by a device equippedwith an LED light source where otherwise the material possesses only lowvisibility to the unaided eye.
 7. The method according to claim 1,wherein the chiral nematic liquid crystal material includes afluorescence dye, a fluorescent laser dye, a quantum dot, or other lightharvester or gain additives.